Only 1 question.
What happens if this grand plan cools the earth too much? How do they propose to reverse it?
Jerry Pournelle and Larry Niven wrote a novel exploring this very idea (i.e. what if the enviros got everything they wanted?). The novel is called Fallen Angels, and is available online for free.
Herein lies the problem.
Their models, as advanced as they are, do not (read: can not) incorporate everything which goes into predicting the future of the climate.
Therefore, at best, their models are educated guesses. At worst, they are pure speculation